Radar moving target indicating system



May 20, 1952 w. M. HALL ETAL RADAR MOVING TARGET INDICATING SYSTEM Filed April 17, 1947 /NVENm/s W/LL/AM M. HALL HAROLD BY /V BEVER/DGE OPNEV illustrati,on of a preferred form Patented May 20, 1952 RADAR MOVING TARGET INDICATING SYSTEM` William M. Hall, Lexington, andv Harold N.

Beveridge, Arlington, Mass., assignors to Raytheon Manufacturing Company, Newton, Mass.,

a corporation of Delaware Application April 17, 1947; SerialiNo. '742;048

(ci. 34e- 9.5)

4 Claims. 1

The present invention relates to an improved radar receiving and indicating. system for si'- multaneously and clearly indicating on a single indicating device the position of both xed and moving objects within the range of the system.

It is an important object` of this invention to provide an improved form of radio direction and ranging system having means for producing standard indications in one optical identifying sense of both iixed and moving targets or objects together with a second means for indicating in another optical identifying sense the moving targets or objects only, the system further including means for optically superposing the two aforesaid indications in a` manner to give a single, clear andeasily readable indicationof the moving objects in the presence of large masses of indications representative of fixed` land objects or ground clutter within the range of the system.

It is a serious disadvantage of known radar receiving and indicating systems that the indication of moving objects in the presence of large masses of indications of xed objects or ground clutter" is very difiicult to obtain due to the apparent merging of the moving object indications with the xed object indications on the fluorescent screen of the indicating cathode ray tube. The system of this invention provides for the simultaneous operation of a standard form of radar receiving circuit for producing video l,

circuit, in the system of the invention, is applied to individual indicating cathode ray tubes which may preferably be of the plan position indicating type and the resultant visual indications are optically combined through means of a halfsilvered mirror or the like to produce a single combined visual indication. By interposing suitably diierent color lters in front of each plan position indicator, as described by this invention, the combined visual indication will enable; the presence of moving objects to be readily detectable andV indicated even in the presence of large masses of indications of fixed or land objects.

Other objects and advantages of the invention will be apparent with reference to the following, specification and drawing in which the single figure is a combined diagrammatic and schematic of the system of 'thfinvention lj., Referring'to f the drawing',l a radar transmitterv I0, preferably having an operating frequency of 3300A megacycles per second, is provided. A suitable pulse interval or repetition rate for the pulse output of the radar transmitter may be provided and. for purposes of this description, the radar system is intended to operate Witha pulse interval of 500 microseconds and a pulse duration ofapproximately one microsecond. The means for accurately triggering the transmitter I0 to predetermine the pulse interval will be later referred to in connection with the description ofthe special form of radarv receiving system for indicating moving targets or objects only. The pulse output from the transmitter I0 is applied by the transmission line II to the combined radiating and receiving antenna I2 of any suitable known design which may be mechanically rotated about in azimuth. Transmitted pulses in the transmission line I I are prevented from entering the standard radar receiving system generally shown at 20 by a conventional form of transmit receive switch I3. Transmitted pulses in the transmission line I I are eiiectively applied, however, to the special form of moving target radar receiving and indicating system generally shown at 50 through the medium of an attenuating circuit I4. The attenuating circuit [4 reduces the power of the transmitted pulse so as to prevent damage to the input circuits of the movingtarget radar receiving and indicating system 50 and further attenuates the reected pulses received at the antenna I2 to such an extent that they are imperceptible at the aforesaid input circuits.

Reected pulses indicative of both xed and moving objects withinrthe range of the antenna I2 at any given instant are applied through the transmit receive switch I3 to a mixer circuit I5 where they are combined with a signal from the local oscillator I6 having a suitable operating frequency of 3270 megacycles per second. The intermediate frequency pulse output of 30 megacycle frequency thus produced from the mixer I5 is suitably amplified at I6, detected at I1 and applied as a video voltage pulse to a plan position indicating cathode ray tube I8. One form of plan position indicating tube I8 may be of the type shown to have a continuously rotated sweep coil I9 synchronized in rotation with the rotation of the antenna I2 as it is rotated about in azimuth. Thus, indications on the screen of the plan position, indicating cathode ray tube I8 will be obtained i/inchare indicative of all objects Within thefrange ofthev antenna `I2 and', as is conventional in suchv systems-'the fluorescent screen. In order to facilitate the identication j of a moving ob-ject when it moves into the range of indication of large masses of iixed objects such as ground clutter or the like, the system of the invention provides for a simultaneous separate indication of moving objects only which may be viewed on the uorescent screen of a separate cathode. ray tube in a different color and Voptically superposedover the standard form of indications.

'There are anmber of known forms of special pearing across load resistance 60 to ground. The cathode of diode 55 is connected to the plate of diode 56 through a center-tapped output load resistance 6I and suitable iilter condensers 62 and 63 are connected from each end ofthe center-tapped output resistance 6I to ground. The reiiected pulse signals of an intermediate frequency of 30 megacycles per second in line 53 are applied to the balanced modulator circuit 54 through the primary winding 64 of the input coupling coil and are inductively coupled to the secondary winding 51 previously referred to. In the absence of any reflected pulse signals in the e primary winding 64, the continuous wave reftypes' of radar receiving systems for indicating Y.

moving targets or objects only and, in the moving target radar receiving system 50 described in connection with the indicating system of this invention, a separate reference oscillator I having a frequency of megacycles per second is provided. The reference oscillator 5I is of the continuous wave typerwhich is, however, continuously locked in phase Ywith the phase of each transmitted pulse from the radar transmitter Il). In order to lock the continuous wave reference oscillator 5I in phasewith the phase ofY each `transmitted radar pulse, a sample of each transmitted pulse is applied through the attenuator circuit I4 to the mixer circuit 52 for combining the transmitted pulse with the output of the local oscillator I6 and producing an attenuated intermediate pulse frequency of 30 megacycles per second in line 4I. V'Ihe intermediate pulse frequency of 30 megacycles per second inline 4I is then capacitively coupled to the grid circuit of the continuous Wave reference oscillator 5I or in any other suitable manner of connection for locking the phase of the reference oscillator with the phase of each transmitted pulse. The output of the continuous wave reference oscillator appearing in line 58 is thus continuously locked in phase with the phase of each succeeding-transmitted radar pulse from the transmitter I0 and is then to be compared in phase to the phase of a reflected pulse as obtained from the intermediate frequency amplifier I6 andA applied through line 53 to the balanced modulator.54.

The balanced modulator 54 may be of any type which will produce a voltage output that is indicative in amplitude and polarity of the phase and amplitude relations between two separate electrical quantities of the same frequency applied to its input circuits and which further will produce zero output voltage when only one of the two electrical quantities is applied to its input circuits. The balanced modulator 54 in the system o-f this invention may preferably be of the type employing a pair of diodes 55 and 56 connected as shown. The plate of diode 55 is connected to one end of the center-tapped input coupling coil secondary winding 51 While the cathode of diode 56 is connected to the other end of the secondary winding 51. The secondary winding 51 is center-tapped at 5S and the output from the continuous wave reference oscillator. 5I.

erence oscillator signal in line 58 as applied to the center tap 59 of the secondary winding 51 will produce equal and opposite voltages at each end 'of the center-tapped output load resistance 6I which effectively cancel each other at the center tap and produce zero voltage output in Ithe'line 65 from the balanced modulator154.V YIn the Apresence'of aV reflected pulse signal'in line 53 andapplied to theprimary Winding 64 of the input coil for the balanced modulator 54,l a` volt# age pulse indicative in amplitude and polarity of the instantaneous phase amplitude relations between the reilected pulse signal in line 53 and the continuous `Wave reference oscillator signal in line 58 will be obtained in line 65.

The voltage pulses in line 65 are then employed toV amplitude 'modulateV at 66 the 'output signal of a frequency of ten Ymegacycles per second from the local oscillator 61. T he lamplitude modulated output signal thus obtained in line 68 is then applied tov a delay line 69 which may be of the mercury type or any other suitable type having a 500 microsecond delay equal tothe radar transmitter pulse interval and functioning to thus vdelay any periodically varying electrical energy having a frequency of 10 megacycles per second applied thereto. The amplitude modulated ten-megacycle signal in line 68, indicative of the phase relations between the reflected pulse signal in line 53 and the signal output of the continuous Wave reference oscillator locked in phase with the transmitted pulse, vis also applied through line10 to one input terminal of a comparison detector circuit 1 I The other input terminal of the comparison detector circuit 1I is connected through line 12 to the output terminal of the delay line 69. The comparison detector circuit 1I consists Aof the pair of diode tubes 13 and 14 connected as shown. Diode tube -13 is connected as a positive rectifying detector while diode 14 is connected as negative rectifying detector and both detecting diodes 13 and 14 may be tuned to the ten-megacycle per second carrier frequency of the signal to be detected by the input coils 15 and 16. The positive and negative rectier output signals from diodes 13 and 14`are connected together through a center-tapped output load resistance 11 such that equal amplitudes of detected signals by the tubes 13 and 14 will cancel each other at the center tap to produce zero voltage signal output in line 13. If the amplitudes of the detected signals applied to the input terminals of the respective detector diodes 13 and 14 are not equal, however, an output voltoutputvoltage pulseof any polarity isobtained.

'I'he cathode ray tube 80 is similar to the cathode the purposes of'this description has been chosen to be, l500 microseconds, a blocking oscillator 8| is provided normally having a repetition rate of between two and three kilocycles per second` The outputtrigger pulse from the blocking oscillator 81|! is applied by line 82 to trigger the radar transmitter I0,A and is also appliedv by line 83 to` theI input terminal 84 ofthe delay line 69. 'Iheblocking oscillator output pulse in line 83 applied' to the delay line `|59 appears at the output' of the delay line after a time delay of 500 .microseconds and is applied as a delayed synchronization-pulse in `lines 84 and 85 for again triggering the blocking oscillator 8| at atwokilocycle. repetition. rate to produce subsequent trigger pulses inline 82. Thus the trigger pulses inline 82V provide a pulse interval for the radar transmitter |'0 exactly equal to the delay line interval of 500microseconds. v

.Considering now the operation of the moving Atarget or' object 'radar receiving and indicating system- 50, it` will be seen that voltage pulses indicative in amplitudeand polarity-of the phase relations of the Ireflected-.pulses in line 53 to the continuous ,Wave reference oscillator signal in line 58 will appear inline B5. It should also be borne in mind that the continuous wave reference oscillator as previously described, is continuously locked in phase with the phase of eachn transmitted pulse from the transmitter ||l so that, in effect, the pulse amplitudein line 54 is indicative of the instantaneous phase relation between each reflected pulse and each succeeding transmitted pulse. Assuming the reflected pulse tobe indicative of a xedobject only and that the position of radar receiving and indicating system is also fixed, it will be seen that the pulse amplitude and polarity in line 65 will always be the same. However, should the reflected pulse be indicative of a moving object under such conditions the phase relation of the reflected pulse in line 53 to the continuous wave reference oscillator signal in line 58 will be changed considerably with each successive pulse and, therefore, there will be considerable variations in pulse amplitude or polarity for each successive voltage pulse in line 64. Again assuming the reflected signal to be indicative of a fixed object only, it will be understood that the amplitude modulated signal o-f a carrier frequency of ten megacycles per second in line B8 will have the same amplitude modulation envelope for each succeeding pulse representative of a reflected pulse from the xed object. ITherefore, the amplitude modulation envelope in line 10 indicative of the phase relation of a first reected pulse will be equal at any given instant to the amplitude of the modulation envelope indicative of the phase relation of a preceding reflected pulse as delayed by the delay line 69, and when the equal amplitudes of the two signals in the respective lines 10 and 12 are positively and negatively rectified and compared by the comparison detector circuit 1|, `the resultant voltage or video output signal in line 18 will be zero. Thus, in. such manner, the

presence of a fixed object results in a zero video voltage in line 18 and causes the plan position indicating cathode Vray'tubel 80 to give no visual indication of such fixed object. If, on the other hand, the reflected signal pulses in line 53. are indicative of a moving object, the amplitude of each succeeding pulse signal inline 65 will be considerably diferentand may even have adifferent' polarity .due to the change in phase relations between each succeeding reiiected pulse in line 53 relative to the continuous wave reference oscillator signal in line 53, as previously described. Thus, the amplitudemodulation envelope f or the local'oscillator signal from the ten-megacycle local oscillator 61 as appearing in line 68, representative of each succeeding reflected pulse, will be different when indicating a moving object and an: output voltage or video signal will appear in line 'I3 from the output of the comparison of detectorv circuit 1| for indicating such moving object on the plan position indicating cathode ray tube 80. In other words, the amplitude of the modulation envelope in line 'l0 indicative of one phase condition of a reected pulse from a moving object will be different from the amplitude ofV the modulation envelope in line 'l2 indicative of av different phase condition of a preceding reflected pulse from a moving object, and with the respective positive and negative rectifying detectors 13 and 14 connected as shown, a voltage output video signal will appear in line` 18 indicative of the amplitude difference` between said two amplitude-modulated signals in lines 1G and 12, respectively.

In view of the foregoing description, it will now be understood that the indication on the screen of the` plan position indicating cathode ray tube 80 is representative of a moving target or object only Within the radar system range while the indication on the` standard radar receiving circuit and plan position indicating cathode ray tube i8 is indicative of both fixed and moving objects. In order to enable the presence of a moving object to be seen when it moves within the field of the fixed objects, the visual indications from the screens of both indicating tubes may be optically combined through the medium of the half-silvered mirror 98. As shown in the drawing, the indicator tube I8 may be positioned at right angles to the indicator tube 80 and the half-silvered mirror 98 may be positioned at a 45 angle between the viewing ends of the two tubes. Suitably different color lters 9U and 92, which may be red and green lters, respectively,

' are interposed between the half-silvered mirror 9|] and the respective indicator tubes IB and 88 to produce diierent optical senses for the indications `from each of the tubes. Thus when viewing the optically combined visual indication in the direction of the arrow on the drawing, all fixed objects will be indicated in red color while moving objects will be indicated in a combined red and green or, optically speaking, a yellowish color appearing on the red background of the fixed objects. It should be understood that other choices of color filters or optical identifying senses for the respective indicating tubes I8 and 80 may be employed without departing from the spirit of the invention. For instance, assuming that both indicator tubes |8 and 8B have a white light emitting uorescent screen, it is entirely .practical to provide only the red color filter in front of'the indicator tube i8 in order to present the visual indications 0f both tubes I8 and 80 in different optical color senses. Similarly,

7 the indicator tube 80 may be provided with a blue light emitting fluorescent screen While the indicating tube I8 may be provided with a White light emitting fluorescent screen in which case neither color iilter is needed.

While the invention has been described in connection with one form of moving target or object receiving and indicating radar system of the type employing a continuous Wave reference oscilla.- tor, it should be understood that other forms of moving target receiving and indicating systems such as the type employing reflections from fixed objects as a reference signal may also be used when providing for a simultaneous single presentation of both iixed and moving objects in the manner of this invention.

What is claimed is:

1. A radar receiving system comprising: a first circuit for detecting a first group of signals corresponding to all of the objects Within Athe range of the system; means, receptive of said first group of signals, for producing visual indicia thereof; a second circuit for detecting a second group of signals corresponding only to the moving objects within the range of the system; means, receptive of said second group of signals, for producing visual indicia thereof; and means for viewing the visual indicia corresponding to said first and second groups of signals simultaneously in superposed relationship.`

2. A radar receiving system comprising: a rst circuit for detecting a first group of signals corresponding to all of the objects Within the range of the system; means, receptive of said first group of signals, for producing visual indicia thereof; a second circuit for detecting a second group of signals corresponding only to the moving objects Within the range of the system; means, receptive of said second group of signals, for producing visual indicia thereof; means for viewing the visual indicia corresponding to said first and second groups of signals simultaneously in superposed relationship; and iiltering means of contrasting colors interposed, respectively, between said first and third-named means, and said second and third-named means.

3. A radar receiving system comprising: a iirst circuit for detecting a rst group of signals corresponding to all of the objects within the range of the system; a first cathode ray tube, receptive of said first group of signals, for producing visual indicia thereof; a second circuit for detecting a second group of signals corresponding only to the moving objects within the range of the system; a second cathode ray tube, receptive of said second group of signals, for producing visual indicia thereof; said cathode ray tubes being disposed with their viewing ends at right angles to each other; and a half-silvered mirrorv disposed at a angle with respect to the viewing ends of said cathode ray tubes for viewing the visual indicia corresponding to said first andr second groups of signals simultaneously in superposed relationship.

4. A radar receiving system comprising: a. first circuit for detecting a first group of signals corresponding to all of the objects Within the range of the system; a first cathode ray tube, receptive of said first group of signals, for producing visual indicia thereof; a second circuit for detecting a second group of signals corresponding only to the moving objects within the range ofthe system; a second cathode ray tube, receptive of said second group of signals, for producing visual indicia, thereof; said cathode ray tubes being disposed with their viewing ends at right angles to each other; a half-silvered mirror disposed at a 45 angle with respect to the viewing ends of said cathode ray tubes for viewing the visual indicia corresponding to said first and second groups of signals simultaneously in superposed relationship; and filtering means of contrasting colors interposed, respectively, between said first cathode ray tube and said mirror, and said second cathode ray tube and said mirror.

WILLIAM M. HALL, HAROLD N. BEvERmGE.

REFERENCES CITED The following references'are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,251,984 Cleaver Aug. 12, 1941 2,408,050 De Rosa Sept. 24,v 1946 2,408,848 Hammond Oct. 8, 1946 2,426,979 Ayres Sept. 9, 1947 2,434,897 Ayres Jan. 27, 1948 2,512,144 Emslie June 20, 1950 2,514,828 Ayres l July 1l, 1950 

